The only mystery fish William Beebe and Otis Barton witnessed twice was the pallid sailfin, or the Bathyembryx Istiophasma. Their first sighting, sometime in 1934 was just after they were being swung around in the turbulent waters 1500 ft below the surface.
Suddenly, they witnessed four bizarre elongated two-foot fish, unlike any previously known species. They quickly swam through their spotlight. One managed to stay in view, remaining motionless besides its waving fins. The fish, which neither scientist could tell if it was an eel or not, lacked any sort of bioluminescence, had small eyes, a large mouth and long, thin pectoral fins and drab, olive coloured skin. The most unusual detail, however, was that they seemed to almost lack a tail. Strangely, the caudal fin was simply a vestigial, fleshy growth. It seemed to use its vertical fins instead as the primary propulsion. These vertical fins were very large, being almost as long as its body.
The pallid sailfin is considered to be one of the most striking of Beebe’s abyssal fishes, as few other species resemble it, perhaps only a couple of species in the Cetomimidæ genus.